Reference may be made to moisture sensitive materials based on fluorescent molecules wherein this type of materials can be used to sense moisture by monitoring their optical changes. J. Kunzelman, B. R. Crenshaw, C. Weder, J. Mater. Chem. 2007, 17, 2989. The drawbacks are fluorescence of the film is quenched and the process is irreversible.
Reference may be made to U.S. Pat. No. 3,618,765 wherein currency counterfeit detector is used to detect the chromamorphic response and magnetic characteristics of US paper currency. The drawback of this invention is the counterfeit also incorporates the chromamorphic response and magnetic characteristics.
Reference may be made to U.S. Pat. No. 5,662,735 wherein chemical solution containing iodine is used to detect currency counterfeit. The drawback of this invention is the counterfeit also incorporates this property of chemical solution of iodine and thereby making it difficult to differentiate from the genuine currency.
Reference may be made to U.S. Pat. No. 5,757,001 wherein detecting currency counterfeit by measuring the reflectance of near infrared beams and comparing the reflectance values with those from genuine currency. The drawback of this invention is the property of reflectance of near infrared beams can be incorporated to fake currency making it difficult to differentiate from genuine currency.
Reference may be made to self-erasable and rewritable materials in which information is written over metastable nanoparticle “inks” wherein these materials can be useful for storing sensitive or temporary information are known from the work of R. Klajn, P. J. Wesson, K. J. M. Bishop, and B. A. Grzybowski, Angew. Chem. Int. Ed. 2009, 48, 7035 and US patent 20100328410A1. The drawbacks are written images are visible in normal day light and light of certain wave length is used to write the images.
Reference may be made to U.S. Pat. No. 7,315,876 filed by Xerox Corporation wherein self-erasable printing paper allows recycling of paper by using a polymer incorporated photochromic molecule for printing on paper. The drawbacks of this invention are written images are visible to eyes under normal light and images are printed on paper using light of certain wave length.
Reference may be made to rewritable photonic paper with hygroscopic salt solution as ink was made by J. Ge, J. Goebi, L. He, Z. Lu, and Y. Yin Adv. Mater., 2009, 21, 4259. The drawbacks of this invention are the writing made by the hygroscopic salt solution as ink is not self-erasable.
Reference may be made to rewritable phosphorescent paper by the control of competing kinetic and thermodynamic self-assembling events was made by A. Kishimura, T. Yamashita, K. Yamaguchi and T. Aida Nature materials, vol 4, July 2005. The drawbacks of this invention are writings made at particular temperature and erasable using different rate of temperature.
Reference may be made to mechanochromic fluorescent materials as a candidate for writing and imaging by G. Zhang, J. Lu, M. Sabat and C. L. Fraser, J. Am. Chem. Soc. 2010, 132, 2160 and S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi, D. Kim and S. Y. Park, J. Am. Chem. Soc. 2010, 132, 13675. The drawbacks of this invention are writings made by applying force and reusability can be done using thermal annealing or exposing to solvent vapors.
Reference may be made to Oligo (p-phenylenevinylene) derivatives that are known to form self-assembled structures in polystyrene matrix which change the fluorescence from green to blue upon heating and hence is useful for thermal imaging by S. Srinivasan, P. A. Babu, S. Mahesh, and A. Ajayaghosh J. Am. Chem. Soc. 2009, 131, 15122. The drawbacks are writings are made using thermal pen which is not friendly to use and written images are not self-erasable.
However as evident from above references self-erasable hand writing using water as an ink is unknown, the fluorescent compound having formula 1 that changes fluorescence color when in contact with water is not known, use of formula 1 for security application is not known.